In an MRI apparatus, a superconducting magnet is used as a generation source of a strong static magnetic field. The superconducting magnet forms a superconducting coil for which a superconducting wire is wound on a coil bobbin and has a configuration fixed by filling the interspaces of the superconducting wire with resin. The superconducting magnet is cooled to a temperature (normally, for example, 4.2 Kelvin, the boiling point of liquid helium) at which the superconducting magnet transitions to a superconducting state; an electric current is applied to the superconducting coil to attain a rated magnetic field after cooling the superconducting magnet; and a circuit referred to as a superconducting switch is closed to reach a closed-loop state where a permanent current flows. Hence, the superconducting state can be maintained.
However, as described in Patent Literature 1, in a permanent current mode, if a superconducting wire of a superconducting coil is moved a few nm for some disturbance or if a crack is formed in resin fixing the superconducting wire, local heat is generated. If the temperature of the superconducting wire exceeds the critical temperature due to the heat generation, transition (quench) from superconduction to normal conduction is generated. If quench is generated, a large amount of liquid helium is consumed; liquid helium reinjection is required in order to activate the superconducting magnet again, which results in a loss of time and human resources.
Therefore, in Patent Literature 1, it is presumed that “secular change” of a superconducting coil is the cause of a superconducting wire being moved and a crack in resin, and the technique to accelerate a secular change of the inner structure of the superconducting coil substantially in advance by repeating magnetization and demagnetization of a superconducting magnet and applying an overcurrent is suggested. Hence, sudden quench hardly occurs during a period while a permanent current for a long time has been kept.
On the other hand, an open-type MRI device that does not provide a patient to be an object with a sense of limitation is known. In the open-type MRI device, an annular helium vessel in which liquid helium is filled is disposed symmetrically on the top and bottom across a space in which an object is to be disposed, and coil bobbins on which superconducting coils are wound are respectively accommodated in the vessel. In Patent Literature 2 and 3, an example of the coil bobbin structure in an open-type MRI device is disclosed.